Combustion effluents and waste products from various installations are a major source of air pollution when discharged into the atmosphere. One particularly troublesome pollutant found in many combustion effluent streams is NO.sub.2, a major irritant. Furthermore, it is believed that NO.sub.2 undergoes a series of reactions known as photo-chemical smog formation, in the presence of sunlight and hydrocarbons. The major source of NO.sub.2 is NO.sub.x which to a large degree is generated at such stationary installations as gas and oil-fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass furnaces, cement kilns, and oil field steam generators.
Various methods have been developed for reducing the concentration of nitrogen oxides in combustion effluents. One such method is the non-catalytic method disclosed in U.S. Pat. No. 3,900,554 to Lyon, which patent is incorporated herein by reference. The process disclosed in that patent teaches the reduction of NO.sub.x to N.sub.2 by injecting ammonia into the combustion effluent stream at an elevated temperature. In general, the following two equations describe the reactions which govern the overall process: EQU NO.sub.x +NH.sub.3 +O.sub.2 +(H.sub.2).fwdarw.N.sub.2 +H.sub.2 O EQU NH.sub.3 +O.sub.2 .increment.NO.sub.x +H.sub.2 O
As indicated by the first equation, hydrogen (H.sub.2) can be injected along with NH.sub.3 to extend the effectiveness of the first reaction, for example, at lower temperatures. Of course, it is desirable to minimize the formation of NO.sub.x according to the second equation.
U.S. Pat. No. 3,900,554 teaches the use of ammonia either as a pure substance or in a precursor form. Useful precursor forms of ammonia include the compounds ammonium carbonate, ammonium formate, and ammonium oxalate. All the these substances yield ammonia on vaporization, while the formate and oxalate precursors also yield formic acid and oxalic acid respectively. Vaporization of the ammonia or its precursor may be accomplished as a separate step or by its injection into the hot effluent being treated. If vaporization of ammonium formate or ammonium oxalate, or their solutions in water, is accomplished as a separate step, then one may, if desired, decompose the formic, the oxalate acid, or both, by either thermal or catalytic means prior to injection into the hot effluent.
Since the issuance of U.S. Pat. No. 3,900,554, there has been a proliferation of patents and publications relating to the injection of ammonia into combustion effluent streams for reducing the concentration of NO.sub.x (nitrogen oxides). The present invention builds on and is a further improvement to the teachings of Pat. Nos. 3,900,554, 4,115,515, 4,423,017, 4,507,269, 4,624,840 and 4,636,370. Although it has generally been disclosed that ammonia or its precursor may be stored and/or used in a solution of water, the process as defined by the above-mentioned patents has been commonly practiced via the injection of vaporized anhydrous ammonia. However, there are some perceived problems with using anhydrous ammonia for this process. Although anhydrous ammonia is a commonly used commodity, there has been growing environmental and safety concerns regarding the storage of large amounts of ammonia at plant sites. The use of aqueous ammonia alleviates these concerns, since it can be stored at atmospheric pressure and, should there be a spill, the ammonia release will be slowed to the extent that it will not represent nearly as significant a threat to human health as would the release of anhydrous ammonia.
Although posing less of an environmental risk, the use of a liquid aqueous form of ammonia has been problematic because conventional injection in such form requires excess ammonia, in order to overcome the inherent maldistribution of ammonia resulting from the rapid vaporization of the ammonia solution upon injection. As a result, either additional chemicals are needed to reduce ammonia slip and/or additional ammonia removal equipment, in the flue gas cleanup system, are needed to reduce the excess ammonia to acceptable levels.
This invention overcomes, or substantially decreases, the limitations of conventional or existing practices. This is accomplished by the use of an aqueous solution of ammonia to produce an ammonia-containing vapor for injection into a combustion effluent. The use of a vapor greatly improves mixing, thereby reducing to a minimum the the amount of excess ammonia.
Vaporization of an aqueous solution of ammonia to produce an ammonia containing vapor, however, results in a blow-down stream that, although low in ammonia concentration, may contain too much ammonia to be directly disposed of in an environmentally acceptable manner without further treatment. A further aspect of this invention involves the disposal of this blow-down stream in an environmentally acceptable manner.